Sunday, December 10, 2006

Classic papers in genetics and evolution that are available in Pubmed Central - Paper 1 - Luria and Delbruck on the origin of mutations

I am starting a new series here ---- finding and writing about classic papers in Evolution and Genetics and Molecular Biology that are available for free in Pubmed Central. And though plenty of classic papers are not avilable, a good collection of them are.

My first selection is Luria and Delbrück's paper from 1943 on the origin of mutations. This papers is near and dear to my heart in many ways and I still remember looking it up for the first time when I was an undergraduate. I was taking a class from Jennifer Doudna on the origin of life, and we had to write a paper as part of the class. In the class we had discussed a new paper by John Cairns and colleagues that revisitid the origin of mutations question. Cairns et al. result suggested to them at least that bacteria could pick and choose the mutations they needed for increased fitness in a particular situation (suggestive of so-called Lamarckian evolution to them). I was fascinated by this work (so much so, that it became the topic of my grad. school application essays and the topic of my first two years of PhD research in Phil Hanawalt's Lab). So I chose to write a paper on the origin of mutations.

Obviously, to write such a paper I had to go to Luria and Delbrück's work since they were the first to experimentally test the question of how much mutations pre-existed selection and how much they arose after selection. I stillremember sifting through the old journals in the library at Harvard to find this and turning the pages in teh dust covered volume to read it.

Luria and Delbrück chose as their experimental system resistance to bacteriaphage and their model organism, Escherichia coli. In their paper first the describe the theoretical underpinning of their wor. In their theory they come up with a way to do an experiment to test whether mutations pre-exist selection or arise in response to it - something now generally known as a fluctuation test. Basically the idea is simple. Take a particular form of the bacteria. Innoculate multiple test tubes with a small amount and let each test tube grow up to a dense culture. Then expose bacteria from each tube to the selective pressure. If the mutations pre-exist selection then there should be big differences between the replicate test tubes in the number of mutants since in some the mutation would arise early in the growth of the culture and in some they would arise late (this is known as a jackpot distribution). If the mutations arise after selection then each tube should have somewhat similar numbers (with some variation around a mean).

And when they did the experiments the results followed very closely the jackpot model. They stated:

We consider the above results as proof that in our case the resistance to virus is due to a heritable change of the bacterial cell which occurs independently of the action of the virus. It remains to be seen whether or not this is the general rule. There is reason to suspect that the mechanism is more complex in cases where the resistant culture develops only several days after lysis of the sensitive bacteria.

And thus they showed that mutations unequivocally pre-exist selection. Now, it turns out that their experiment had a flaw - since they were using a lethal selection (phage) the system did not really allow for a long period of time for the mutations after selection to arise (when your dead it is hard to generate mutations). The Cairns experiment (and Ryan's before him) showed that some unusual results occurred if you used a non lethal selection. So one cannot really use Luria and Delbrück's experiment to disprove the possibility that mutations arise after selection. Nevertheless, their proof that mutations can exist before selection was a fundamental discovery. And their methods were used throughout bacterial genetics for years (to this day in fact). For more detail see Access Excellence page about Luria and Delbruck.

This is such a fundmentally important paper, and it is great that this paper is available, free to all, in Pubmed Central. See below:

I teach a course on Molecular Genetics using "classic" papers, some are easier to find than others. PNAS is nice and they have a section on "classics", not too many yet, but I hope that they will have more in future. I'll use some of these hints/links thanks. -Ken